1. Field of the Invention
The present invention relates to devices comprising an electronic compass, which comprise electromagnetic sensor means for measuring at least two field components of an external magnetic field, and a processing unit, which is arranged to determine the compass direction on the basis of the measured field components, as well as a suitable method for such devices for determining a compass direction. In particular, the invention relates to wristop devices, but it is also well suited for use in fixed applications, such as in vehicles.
2. Description of Background Art
Electronic compasses are used to show the direction in, for example, cars and in wristop computers. In such compasses there are typically two magnetic-field sensors detecting magnetic-field components at right angles to each other. One such arrangement is disclosed in U.S. publication Pat. No. 6,817,106.
The sensors are naturally sensitive not only to the Earth's magnetic field, but also to other external magnetic fields and fields induced by the device itself. Magnetizing substances in the vicinity of the device will also shape external magnetic fields, which can also cause deviation in the reading of the compass. Due to such interfering factors, the compass must be calibrated before the actual definition of direction, so that its reading will be accurate. The calibration is thus mainly intended to eliminate the effect of static disturbance factors, arising from the operating environment, on the definition of direction. In the actual direction definition, a problem is, on the other hand, measurement noise, which is mainly caused by the magnetic sensors being tilted away from the horizontal plane. The signal transmitted by them will then not correspond to the real compass direction, but instead a method will be required to take this noise into account. U.S. publications Pat. No. 6,356,851 and 2002/0035791 describe the problem field relating to the calibration of compasses and some methods for performing the actual calibration in vehicle compasses.
The method disclosed in U.S. publication Pat. No. 6,356,851 is based on searching for the minima and maxima of the sensor readings, the algorithm requiring the device to be moved in such a way that measurement data can be obtained from all the quarters of a circle corresponding to the different directions of the compass. In practice, such calibration is relatively slow and laborious to implement, so that it is not very suitable for use when trekking, for example, and for wristop devices. In order to work reliably, the method also requires a very pure signal.
The method disclosed in US publication 2002/0035791 is based on measuring three pairs of x, y points corresponding to the various orientations of the device and then solving the constants A, B, and C of the equation x2+y2+Ax+By+C=0 on the basis of the measurements. One drawback of the method is that the said three pairs of points should be relatively far from each other (i.e. so that the triangle formed by them has essentially no obtuse angles), so that the device must be rotated to a relatively great extent in order to make a successful calibration. In addition, the method is mathematically quite ponderous, i.e. it consumes a great deal of power. For the aforementioned reasons, the response times also become unnecessary long.
The offset values of the magnetic sensors change through time, so that at some stage the original calibration will give erroneous directional readings. This is corrected by means of calibration made by the user. Even some wristop computers are known, in which the calibration is performed by the device first being set to the calibration state, then rotated 360 degrees in the hand. From the data collected during this rotation, the centre point of a magnetic circle is defined, which is used to calculate the compass direction on the basis of the new measurements, once the device is in the compass state. A problem with this method is that the calibration is slow and labourious while, if calibration is not performed sufficiently frequently, the directional readings given by the device may not be reliable.